1. Field of the Invention
The present invention relates to a magnetic recording medium of a hard disk drive (hereinafter abbreviated as HDD) widely employed in the external memory of computers.
2. Prior Art
At present, a magnetic recording medium used for hard disk drives has reached an areal density of 10 Gbits/in2 on an experimental basis.
Referring to FIG. 10(a), a typical basic layered structure of a magnetic recording medium includes a non-magnetic substrate 1, upon which are deposited, in order, an under layer 2, tens of nanometers thick, a magnetic layer 3, from 10 to 20 nm thick, and a carbon protective layer 4, some 10 nm thick. Non-magnetic substrate 1 is typically a hard substrate made of aluminum alloy or glass, hundreds of micrometers thick. Under layer 2 is composed of pure chromium metal or a chromium alloy doped with molybdenum or tungsten. Magnetic layer 3 employs a cobalt alloy doped with chromium, nickel, tantalum, platinum etc. Under layer 2 seeds crystallinity in magnetic layer 3 formed thereon. Under layer 2, for instance, orients c axes of polycrystalline cobalt, which has a crystallographically hexagonal symmetry, in a magnetic layer 3 parallel to the substrate surface.
It is assumed that a conventional magnetic recording medium with all its layers manufactured by a sputtering technique has a maximum attainable areal density of tens of gigabits per square inch. An areal density of 10 Gbits/in2 requires an area of some 6xc3x97104 nm2 in the magnetic layer to record one bit. In order to reduce noise accompanying the read out of written bit information, the diameter of magnetic metal crystal grains in magnetic layer 3 must be reduced. Reducing crystal grain diameter, however, causes difficulty in retaining written information (the magnetization direction) when the crystal grains are subject to disturbance caused by thermal oscillation. Deliberation on the disturbance caused by thermal oscillation estimates for the present that the minimum crystal grain diameter enabling stable retainment of the magnetization direction is some 9 nm. This deduces that some 1000 crystal grains at an areal density of 10 Gbits/in2 hold one-bit information, with crystal grain diameter minimized as far as thermal stability of the crystal grain permits.
The magnetic layer of a magnetic recording medium with a high areal density most conventionally uses cobalt magnetic alloy. Crystal grains of cobalt magnetic alloy have a hexagonal crystallographical structure. A magnetic layer formed by sputtering has non-magnetic crystal grains segregated at grain boundaries with magnetic crystal grain. Hence, the cobalt magnetic alloy has a crystalline structure in which magnetic metal crystal grains are surrounded by non-magnetic metal, such as chromium.
The presence of the non-magnetic metal among the magnetic metal crystal grains reduces interaction among magnetic crystal grains compared with those in the absence of non-magnetic metal. This reduces noise in the medium.
The above suggests that reducing interaction among magnetic crystal grains in a magnetic layer enables spontaneous reversal of the direction of magnetization of the magnetic crystal grains. Intense interaction among magnetic crystal grains directly contacting each other in a magnetic layer causes difficulty in reversing the magnetization direction of a specific crystal grain when the magnetization direction of surrounding crystal grains remains unreversed.
Referring now to FIG. 10(b), magnetic layer 3 consists of magnetic metal crystal grains 13 and non-magnetic metal 14. In the absence of under layer 2, the directions of the c axes of magnetic metal crystal grains are randomly oriented, as indicated by the random directions of arrows. Under layer 2 (FIG. 10(a)) eliminates the random orientation in c axes of magnetic metal crystal grains 13 and arranges them in a specified orientation. Disturbance caused by thermal oscillation, however, increases randomness of orientation in c axes, when the diameter of magnetic metal crystal grains 13 is reduced.
Referring now to FIG. 11, in order to eliminate the instability in the magnetization direction of magnetic metal crystal grains 13 on reducing diameter thereof, a magnetic recording medium may be modified as shown. As in the embodiment in FIG. 10(a), the medium consists of a non-magnetic substrate 1, an under layer 2, a magnetic layer 3, and a protective layer 4, in that order starting at the non-magnetic substrate 1. Non-magnetic substrate 1 is made of silicon. Magnetic layer 3 is composed of a plurality of spaced-apart magnetic metal crystalline column 15. A silicon oxide film 16 fills the spaces between the magnetic crystalline columns 15 to embed the magnetic metal crystalline columns 15 in the silicon oxide film 16. The silicon oxide film isolates each magnetic metal crystalline column 15 from its neighbors. This type of magnetic recording medium is known as a quantized magnetic disk (hereinafter abbreviated as QMD). A magnetic metal crystalline column 15 surrounded by silicon oxide 15 in the magnetic layer 3 corresponds to one bit.
The shape of the magnetic metal crystalline columns 15 determines their spontaneous magnetic orientation (magnetic orientation in the absence of a magnetic field). When a narrow magnetic metal crystalline column 15 situated in a long and narrow hole of silicon oxide film 16 in a QMD, spontaneous magnetization of the magnetic crystalline columns 15 is oriented perpendicular to the surface of a silicon substrate 1. By changing the shape of the magnetic metal crystalline columns it is possible to orient the spontaneous magnetization of a QMD parallel to the surface of a silicon substrate 1. The long and narrow magnetic metal crystalline columns 15 in the QMD of FIG. 11 have the identical orientation in spontaneous magnetization. That is, all magnetic metal crystalline columns 15 have spontaneous magnetization directed perpendicular to the surface of a silicon substrate 1. This limits magnetization only to upward or downward directions. This takes advantage of the constraint of vertical orientation of spontaneous magnetization to reduce the energy of shape anisotropy of the magnetic domains.
Before the development of the QMD, each crystalline region, which may be composed of 1000 weakly combined magnetic crystal grains, corresponded to one bit in a magnetic recording medium. In a QMD, on the other hand, each magnetic metal crystalline column 15 corresponding to one bit is isolated magnetically from all of its neighbors. The crystal grains in a magnetic metal crystalline column 15 are composed of ferromagnetic metal without the presence of non-metal material. The magnetic forces of the crystal grains combine with each other with large exchange forces to cause the magnetic metal crystalline column 15 to behave like a large single magnetic crystal. As a result, the amount of thermal energy required to reverse the magnetization direction in one bit region of a QMD is larger than is the case with a conventional medium that has magnetic metal crystal grains segregated by non-magnetic metal crystal grains. This provides a QMD with improved thermal stability in magnetization.
It is an object of the invention to provide a QMD with a magnetic metal crystalline column evenly isolated in oxide in a magnetic layer that has increased coercive force through surface magnetic anisotropy.
It is a further object of the invention to provide a material for the magnetic layer of a QMD which exhibits increased coercive force.
It is a still further object of the invention to provide a magnetic recording medium with a magnetic layer whose magnetization is less subject to the materials it contains.
It is a further object of the invention to provide a magnetic layer for a QMD that has improved thermal stability in magnetization as well as improved evenness and orderly distribution of magnetic components therein.
A first magnetic recording medium of the present invention to solve the problem comprises a non-magnetic substrate, an under layer, a magnetic layer, a protective layer. The under layer, magnetic layer, and protective layer are formed in that order one on top of another on the non-magnetic substrate. The magnetic layer consists of a plurality of magnetic components, and an isolating component. The magnetic components are distributed evenly spaced apart in the isolating component. Each magnetic component is composed of a hard magnetic layer having a large coercive force, and a soft magnetic layer having a small coercive force placed side by side. The isolating component is a non-magnetic body.
The two-layer magnetic component may also be an upper magnetic layer of small coercive force on top of a lower magnetic layer of large coercive force, or vice versa.
In another embodiment, the magnetic component mentioned above may include a plurality of magnetic layers alternating between magnetic layers of small coercive force and magnetic layers of large coercive force.
A second magnetic recording medium of the present invention comprises a non-magnetic substrate, an under layer, a magnetic layer, a protective layer in that order. The under layer, magnetic layer, and protective layer are formed one on top of another on the non-magnetic substrate. The magnetic layer consists of a plurality of first magnetic components, and second magnetic components. Each of the first magnetic components is embedded in an even and orderly manner in the second magnetic component. The coercive force of the first and a second magnetic components differ from each other.
The first magnetic component stated above may be made of a body of large coercive force, and a second magnetic component may be made of a body of small coercive force.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.